cccc In radiotheraby, the purpose of using standard photon beam (Flattening Filter; FF) is to convert the forward peaked MV bremsstrahlung photon intensity into uniform intensity pattern for obtaining clinically acceptable beam profile. Recently a number of studies were carried out on existing medical linac by removing the flattening filter to produce the unflat photon beam and demonstrated their feasibility in the implementation of advanced radiotherapy techniques. Flattening Filter Free (FFF) beam have a fundamental physical parameter differences with respect to the standard filter flattened beams, making the generally used dosimetric parameters and definitions not always viable. In this concern, the current paper is a trial to shed further light on studying some dosimetric parameters for use in quality assurance of FFF beams generated by medical linacs in radiotherapy. The main characteristics of the photon beams have been analyzed using specific data generated by a Varian TrueBeamlinac having bothFFF and FF beams of 6 and 10 MV energy. Definitions for dose profile parameters are suggested starting from the renormalization of the FFF with respect to the corresponding FF beam. From this point the flatness concept has been translated into one of “unflatness" and other definitions have been proposed, maintaining a strict parallelism between FFF and FF parameter concepts. In summary, although there are a number of advantages of using a FFF beam especially for advanced radiotherapy techniques there are a few challenges (e.g., criteria for beam quality evaluation and penumbra, establishment of dosimetry methods, and consequences of photon target burn-up) which need to be addressed for establishing this beam as an alternate to the FF beam.